1. Field of the Invention
The present invention relates to an impeller such as a centrifugal impeller, a mixed-flow impeller or the like and turbomachinery including the impeller, and more particularly to turbomachinery for applying energy to a working fluid such as a compressor, a blower, a fan, a pump and the like including centrifugal impellers or mixed-flow impellers.
2. Description of the Related Art
A multi-stage compressor which is a kind of turbomachinery has such a configuration that stages, each including many coaxially attached centrifugal impellers or mixed-flow impellers, and diffusers and return guide vanes which are juxtaposed downstream of the respective impellers, are piled up. In an impeller used in the above multi-stage compressor, a blade is produced by cutting work in many cases. If it is allowed to define a blade-surface shape of a blade included in the impeller as an assembly of linear elements, use of a rod-shaped cutting tool such as a mill or the like will be allowed. In the above mentioned case, a side surface of a working tool is brought into abutment on a part of the blade to be worked as a linear element while rotating it and the blade is cut while sliding it in a direction from the entrance side to the exit side of the impeller or in its reverse direction. Owing to the above, efficienct working is attained. Since a linear element impeller (an impeller including linear elements) is excellent in productivity and workability as described above, the linear element impeller is frequently used in a centgrifugal compressor.
Although adoption of a linear element impeller is an effective method from the viewpoint of production, it is desirable to release a blade for use in an impeller from such a restriction that it is defined as an assembly of linear elements, to let it have a blade surface including a free surface so as to finely control a blade passage flow, in order to fulfill the requirements of the times that impeller performance be further improved. In the following, in the present invention, an impeller in which a blade surface includes a free surface will be referred to as a curvilinear element impeller.
Examples of an impeller that partially includes curvilinear elements are disclosed in Japanese Patent Application Laid-Open No. Sho59-90797/1984 and Japanese Patent No. 4115180. An impeller disclosed in Japanese Patent Application Laid-Open No. Sho59-90797 is an open impeller (hereinafter, also referred to as a half-shrouded impeller as the case may be) that does not include any shroud plate (a side plate) on the side of a shroud of the impeller. Although an impeller disclosed in Japanese Patent No. 4115180 is the same as that in Japanese Patent Application Laid-Open No. Sho59-90797 in that any shroud plate (a side plate) is not included on the side of its shround, it is a half-shrouded impeller with half vane that includes, between two blades, a blade which is shorter than these two blades in entrance-side dimension. Incidentally, an impeller that includes a shroud plate (a side plate) on the side of a shroud is referred to as a closed impeller (hereinafter, also referred to as a fully-shrouded impeller as the case may be).
The impeller disclosed in Japanese Patent No. 4115180 is a curvilinear element impeller. Blades used in the curvilinear element impeller are formed by piling up blades the sections of which are curved in a span-wise direction in the vicinity of leading edges of the blades when an airfoil is to be formed. Owing to the above, accumulation of a low energy fluid onto an area of a blade flow passage is restricted to improve compressor efficiency.
In the above mentioned Japanese Patent Application Laid-Open No. Sho59-90797 and Japanese Patent No. 4115180, improvement of compressor efficiency is promoted by changing the configuration of a part around a blade leading edge from an ever used one in the half-shrouded impeller. In the half-shrouded impeller used in a centrifugal impeller or a mixed-flow impeller, a tip leakage flow generates. On the other hand, in a fully-shrouded impeller, any tip leakage flow does not generate. Thus, it may not be ensured that an optimum curvilinear element impeller which is expected to improve performance in a half-shrouded impeller is obtained due to a difference in flow pattern between blades even when a blade of the shape which has been the best in a half-shrouded impeller is used. That is, the blade shape with which an optimum curvilinear element impeller is obtained may be different depending on the situation.
A method of forming curvilinear elements which is suited for a fully-shrouded impeller may not be definitely established as mentioned above. However, it may be easily imagined that the number of curvilinear element forming patterns which would lead to performance improvement in reality is rather limited for numerous curvilinear element impeller forming methods and hence it becomes desirable to find out curvilinear element forming patterns which would lead to performance improvement.